TECHNICAL PAPER Experimental Investigation of Formability and Surface Finish into Resistance Single-Point Incremental Forming of Ti–6Al–4V Titanium Alloy Using Taguchi Design Mostafa Vahdani 1 • Mohammad Javad Mirnia 1 • Hamid Gorji 1 • Mohammad Bakhshi-Jooybari 1 Received: 3 November 2018 / Accepted: 10 January 2019 Ó The Indian Institute of Metals - IIM 2019 Abstract Considering its low-cost equipment, single-point incremental forming (SPIF) can be appropriate for rapid prototyping of sheet metal parts, since a simple hemi- spherical head tool is usually implemented without any dedicated dies. Incremental forming at elevated tempera- tures assisted by electric current, which is hereafter called resistance SPIF, is an innovative process for forming sheet metals with a high-strength-to-weight ratio and a low formability at room temperature. In the present research, the effect of utilizing various lubricants on both the formability and surface finish of the Ti–6Al–4V titanium alloy sheet is investigated using the resistance SPIF of the truncated cone at different feed rates, vertical pitches, and currents. To this end, the Taguchi design of experiment and the analysis of variance (ANOVA) are employed. A cool- ing water system is designed and applied to the forming tool to improve both its life and the duty cycle of the process. The results show that the formability of Ti–6Al– 4V sheet can be enhanced using the resistance SPIF, which strongly depends on the lubrication condition. In order of significance, the lubricant and current have the most influence on the maximum achievable forming depth and the lubricant, the feed rate, and the vertical pitch have the highest effects on the surface roughness. Based on the ANOVA results, the graphite-based anti-seize compound and the graphite powder are suggested for an appropriate surface finish and the highest formability, respectively, in the resistance SPIF within the range of the considered parameters. Keywords Single-point incremental forming Á High temperature Á Resistance heating Á Lubricant Á Formability 1 Introduction Incremental forming (IF) is considered as a low-cost pro- cess for manufacturing complicated and customized sheet metal parts with a short lead time in small batch produc- tion. This superiority has distinguished IF from conven- tional sheet metal forming processes. IF process can be classified into two types, namely single-point incremental forming (SPIF) and two-point incremental forming (TPIF). There is only a localized contact region between the forming tool and the sheet in SPIF, while during TPIF, the sheet is also in contact with a supporting tool. In SPIF, the hemispherical head tool locally deforms the sheet metal along a predefined tool path according to a desired geom- etry simply using a CNC milling machine [1]. Cold forming of titanium, magnesium, and some alu- minum alloy sheets is hardly possible due to their low formability, high forming force, and large springback at room temperature [2]. The low formability of some of these alloys can be attributed to their limited slip systems at room temperature. On the other hand, it is possible to activate more slip systems by elevating the forming tem- perature [3]. Accordingly, high-temperature SPIF has attracted much attention for rapid prototyping of sheet metal components from the aforementioned alloys. Duflou et al. [4] locally and dynamically elevated the temperature of AA5182 aluminum and Ti–6Al–4V tita- nium sheets during SPIF using a 500 W Nd:YAG laser source. A liquid lubricant was used. A reduction in & Mohammad Javad Mirnia mirnia@nit.ac.ir 1 Mechanical Engineering Department, Babol Noshirvani University of Technology, Babol 47148-71167, Iran 123 Trans Indian Inst Met https://doi.org/10.1007/s12666-019-01577-4